1. Field of the Invention
The present invention relates to a zoom lens that is used, in particular, in an projection lens system for use with a projection type of television set equipped with a liquid crystal display device or as an image forming lens for use with cameras such as an electronic camera equipped with a CCD imaging device, an television camera equipped with a regular optical imaging device and an ordinary film camera.
2. Description Related to the Prior Art
Some of zoom lenses that comprise, in order from the projected image side to the master image side, a negative power first lens group which works as a focusing lens group movable during focusing but is stationary during zooming, a positive power second lens group which works as a zooming lens group, a negative power third lens group which compensates a positional shift of focal point or an image plane during zooming, and a positive power fourth lens group. One of such a zoom lens is known from, for example, Japanese Unexamined Patent Publication No. 5-297276, now patented as U.S. Pat. No. 2,516,522, entitled xe2x80x9cwide Zoom Lens.xe2x80x9d This zoom lens has been designed with the intention to be incorporated to a small size of CCD imaging device. This wide zoom lens system must be large in its own size when used as it is together with an image forming device having a large image size, or this wide zoom lens system is still insufficient in correction of distortion when used as a projection lens. In particular, in the case of using the wide zoom lens in a projection lens system equipped with a liquid crystal display (LCD) device, although the projection zoom lens system is desirably designed to have teleccentricity on a master image side in consideration of illumination optical system, whereas, projection zoom lens systems have been designed without considering such telecentricity at all. Further, many projection zoom lenses have back-focal distances which are too short to permit a color composing optical element or a color separation optical element between the projection zoom lens and an image plane.
In order to eliminate such problems, Japanese Unexamined Patent Publication No. 10-268193 discloses a zoom lens comprising a negative power first lens group which works as a focusing lens group and is stationary during zooming, a positive power second lens element which is movable relative to the first lens group so as to vary a magnification continuously and compensate a positional shift of image plane due to the variation of magnification, a positive power third lens group, a negative power fourth lens group and a positive power fifth lens group which is stationary during zooming and satisfying specific given conditions.
In the recent years, there has been known projectors equipped with LCD elements each of which is provided with a micro lens for increasing an angle at which light emanates from the LCD element so as thereby to collect light effectively with an effect of securing brightness of an image on a projection screen. On the LCD element it is necessary to collect diffraction light effectively. For these reason, although there is a strong demand for a high speed zoom lens, the zoom lens disclosed in the above mentioned Japanese Unexamined Patent Publication No. 10-268193 has an F-number of approximately 2.5 and is consequently hardly capable of answering to the demand for high lens speed. Further, the zoom lens has been designed, though not quite satisfactorily, in consideration of application as a projection zoom lens equipped with a LCD device which I desirable to have telecentricity on the master image side and, however, does not meet a demand for compactness and inexpensiveness because the fifth lens group, which is stationary during zooming, increases its relative diameter when it has a focal point on the projected image side too close to that on the master image side.
It is an object of the present invention to provide a zoom lens that is approximately telecentric on an master image side and compact.
It is another object of the present invention to provide a zoom lens that is well corrected in various aberrations and has a long back-focal distance.
It is still another object of the present invention to provide a zoom lens that distributes light rays parallel to and symmetrical with respect to an optical axis thereof in a tangential plane on an master image side and has a sufficient lens speed while having a wide angle of view.
The above objects of the present invention are achieved by a zoom lens which comprises in order from the projected image side to the master image side a negative power first lens group which is movable as a focusing lens group and remains stationary during zooming, a positive power second lens group, a positive power third lens group, a negative power fourth lens group and a positive power fifth lens group which remains stationary during zooming, said second, third, and fourth lens groups being movable relatively to one another to vary a magnification of the zoom lens continuously and compensate a positional shift of an image plane of the zoom lens due to a variation of the magnification, and satisfies the following conditions (1) to (3):
xe2x80x83xe2x88x921.7 less than F1/F less than xe2x88x920.3xe2x80x83xe2x80x83(1)
0.7 less than F2/F less than 2.2xe2x80x83xe2x80x83(2)
1.5xe2x89xa6F5/F less than 2.2xe2x80x83xe2x80x83(3)
where
F is the overall focal length of the zoom lens at a wide angle end;
F1 is the focal length of the first lens group;
F22 is the focal length of the second lens group;
F5 is the focal length of the fifth lens group.
The second lens group may comprise at least two positive power lens elements and is configured so as to shorten a distance relative to the third lens group as the zoom lens changes its focal length to the telephoto end.
Further, the zoom lens desirably satisfies the following conditions (4) and (5):
0.1 less than D2/F less than 1.2xe2x80x83xe2x80x83(4)
0.05 less than xcex4D2/(FXFt)xc2xd less than 0.6xe2x80x83xe2x80x83(5)
where
D2 is the axial distance between the second and the third lens group at a wide-angle end;
xcex4D2 is an absolute value of a change in the axial distance between the second and the third lens group between the wide-angle end and the telephoto end;
Ft is the overall focal length of the zoom lens at the telephoto end.
The third lens group consists of two lens elements, namely a positive power lens element and a negative power lens element, cemented or separate, and satisfies the following condition (6)
xcexd(xe2x88x92) less than 35xe2x80x83xe2x80x83(6)
where xcexd(xe2x88x92) is the Abbe number of the negative lens element of the third lens group.
According to the zoom lens of the present invention, the second, third, and fourth lens groups are structured movable relatively to one another so as thereby to vary a magnification of the zoom lens continuously and compensate a positional shift of an image plane of the zoom lens that is caused due to the variation of magnification, which reduces a change in aberration for the zoom lens. When structuring the second and third lens groups so as to move close to each other as the zoom lens changes its focal length toward the telephoto end, the distance that the lens groups have to move for zooming can be shortened. Furthermore, aberrations are corrected for the zoom lens as a whole by employing more-than-two positive power lens elements for the second lens group, even when the zoom lens has a high speed, namely a small F-number. In the case where the zoom lens is telecentric on the master image side, if the fifth lens group, which remains stationary during zooming, has a focal point on the projected image side too close to that one the master image side, a lens element of the fifth lens group on the object end generally has to have a large diameter, which are always undesirable for compactness and inexpensiveness of the zoom lens.
As contrasted, while the zoom lens of the present invention that satisfies the conditions satisfactorily balances the correction of aberrations and the back-focal distance, the fifth lens group is designed and adapted to have a focal length within specified limits while balancing the correction of aberrations and the back-focal length for the zoom lens, so as thereby to locate the focal point of the fifth lens group on the projected image side not too close to the master image side.
The conditions (1) to (3) set forth provide a desired distribution of power and necessary for balancing the aberrations of the zoom lens. When the first lens group has an weak negative power exceeding the lower limit of the condition (1), the aberrations are hardly correctable for the zoom lens when it has a small F-number and/or a change in moving distance of the first lens group during focusing is increased, as a result of which increased aberration changes occur. On the other hand, when the first lens group has a strong negative power exceeding the upper limit of the condition (1), the paraxial light rays are leaped by the first lens group, which makes the correction of, in particular, distortion and spherical aberrations. When the second lens group has an weak positive power exceeding the upper limit of the condition (2), a change in moving distance of the second lens group during zooming is increased, which leads to an increased diameter of lens elements of the first lens group. On the other hand, when the second lens group has a strong positive power exceeding the lower limit of the condition (2), the aberrations are hardly correctable for the zoom lens. When the fifth lens group has a strong positive power exceeding the lower limit of the condition (3), the back-focal distance of the zoom lens is shortened so as to be hard to provide the zoom lens telecentricity on the master image side. Besides, the fifth lens group has to have the focal point on the projected image side that is too close to the master image side, which leads to an increased diameter of lens elements on the projected image side of the zoom lens. On the other hand, when the fifth lens group has a weak positive power exceeding the upper limit of the condition (3), the back-focal distance is too long with the result of increasing the overall length of the zoom lens including its flange-focal distance. Further, the distance of incidence of paraxial light rays is too short to correct the aberrations for the zoom lens.
The conditions (4) and (5) set forth provides the positional relationship between the second and third lens groups in order to maintain a compact zoom lens design. Satisfaction of the condition (4) prevents or significantly reduces aggravation of the aberrations, in particular distortion, of the zoom lens. When the distance between the second and third lens group is so short to exceed the lower limit, it is difficult to balance distortion for the zoom lens. Besides, when the distance between the second and third lens group is too long exceeding the upper limit, it is difficult to maintain overall compactness of the zoom lens. When the upper limit of the condition (5) is exceeded, it is difficult to compensate a change in aberration which occurs due to zooming. On the other hand, the lower limit is exceeded, a relative movement of the second and third lens groups for zooming becomes large.
The condition (6) set forth is necessary for ensuring correction of chromatic aberrations. In particular, when the limit is exceeded, chromatic aberrations are hardly correctable.